A typical bumper system for a motor vehicle (passenger car, light truck, sport-utility vehicle, cross-over utility vehicle, etc.) generally comprises a bumper (also commonly referred to as a bumper beam) extending transversely relative to the vehicle and one or more energy absorbing components mounted forward of the bumper. The bumper is typically formed of metal and/or other high-strength material and the energy absorber (EA) is typically made of a foam material and is engineered to deform in order to absorb the kinetic energy of a relatively low-speed impact. A plastic fascia (and/or other trim components) may cover the bumper system to achieve a desired appearance and aerodynamics.
For most passenger vehicles, the EA should be designed to have sufficient stiffness to meet regulatory requirements and/or other targets related to minimizing damage to the vehicle resulting from a low-speed impact. Design constraints (vehicle styling, aerodynamics, etc.) may require that the EA be relatively thin (as measured along the longitudinal axis of the vehicle) and therefore the EA must be made of a relatively stiff material to achieve a sufficient level of resistance to impact damage.
Bumper systems for some vehicles are also designed to meet pedestrian protection requirements/goals related to avoiding/minimizing injuries to a pedestrian when struck by a moving vehicle. A common criterion for pedestrian protection is lower leg injury, and one way to minimize such injury is to reduce the stiffness of the EA so that the portion of the bumper system striking the lower leg is “softer.” Balancing low-speed damageability and pedestrian protection goals may present challenges for the vehicle designer.
It is known to employ magneto-rheological (MR) devices to vary or adjust the stiffness of a vehicle structural component in response to a predicted or actual collision. See, for example, U.S. Patent Application No. 2004/0117086A1. MR devices employ MR fluid which has a shear strength that is negligibly low until it is subjected to a magnetic field, whereupon the shear strength increases by an amount depending on the strength of the magnetic field. Accordingly, controllable electromagnetic devices may be used to apply a magnetic field when desired to achieve a required level of stiffness in the MR device.